Trench mosfet having an independent coupled element in a trench

ABSTRACT

A trench MOSFET is disclosed that includes a semiconductor substrate having a vertically oriented trench containing a gate. The trench MOSFET further includes a source, a drain, and a conductive element. The conductive element, like the gate is contained in the trench, and extends between the gate and a bottom of the trench. The conductive element is electrically isolated from the source, the gate, and the drain. When employed in a device such as a DC-DC converter, the trench MOSFET may reduce power losses and electrical and electromagnetic noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a divisional of Ser. No. 14/666,503filed Mar. 24, 2015, entitled, “Trench MOSFET Having an IndependentCoupled Element in a Trench;” which is a divisional of U.S. patentapplication Ser. No. 13/617,744, filed on Sep. 14, 2012, entitled“Trench MOSFET Having an Independent Coupled Element in a Trench”, whichissued as U.S. Pat. No. 9,000,497 on Apr. 7, 2015. All are incorporatedby reference herein in its entirety and for all purposes as ifcompletely and fully set forth herein.

BACKGROUND OF THE INVENTION

DC-to-DC converters are electronic circuits that convert a directcurrent (DC) source voltage from one voltage level to another. DC-DCconverters are important in electronic devices that contain one or moresub circuits that operate on voltages that are different from itssource. For example, smart phones and tablet computers may contain subcircuits such as central processing units (CPUs), which operate onvoltages that are different than the voltage provided by a source suchas a rechargeable battery. An electronic device such as a smart phonemay contain several DC-DC convertors that produce voltages at distinctlevels for the needs of respective sub circuits. The present inventionwill be described primarily with reference to DC-DC converters employedin portable electronic devices powered by batteries, it being understoodthe present invention should not be limited thereto.

SUMMARY OF THE INVENTION

A trench MOSFET is disclosed that includes a semiconductor substratehaving a vertically oriented trench containing a gate. The trench MOSFETfurther includes a source, a drain, and a conductive element. Theconductive element, like the gate is contained in the trench, andextends between the gate and a bottom of the trench. The conductiveelement is electrically isolated from the source, the gate, and thedrain. When employed in a device such as a DC-DC converter, an inverter,or a motor driver, the trench MOSFET may reduce power loss and/or noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood in its numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 is a block diagram illustrating an example DC-DC convertor.

FIG. 2 is a cross sectional view of a trench MOSFET.

FIG. 3 is a cross sectional view of another trench MOSFET.

FIG. 4 is a circuit diagram illustrating relevant components of thetrench MOSFET shown in FIG. 3.

FIG. 5 is a block diagram illustrating relevant components of a DC-DCconvertor that employs the trench MOSFET of FIG. 2.

FIG. 6 is a timing diagram that shows relevant waveforms of the DC-DCconvertor shown in FIG. 5.

FIG. 7 is a block diagram illustrating relevant components of anotherDC-DC convertor that employs the trench MOSFET of FIG. 4.

FIG. 8 is a timing diagram that shows relevant waveforms of the DC-DCconvertor shown in FIG. 7.

FIG. 9 is a block diagram illustrating a modified version of the DC-DCconvertor shown in FIG. 7.

FIG. 10 is a timing diagram that shows relevant waveforms of the DC-DCconvertors shown in FIG. 9.

FIG. 11 is a diagram of a circuit employing the MOSFET of FIG. 4.

FIG. 12A is a diagram of a circuit employing the MOSFET of FIG. 4.

FIG. 12B shows several wave forms of Vgs of the MOSFET in FIG. 12A withvarying levels of Ves.

FIG. 13 is a diagram of a circuit employing the MOSFET of FIG. 4.

FIG. 14 is a diagram of a circuit employing the MOSFET of FIG. 4.

FIG. 15A is circuit diagram of a DC-DC converter.

FIG. 15B is a timing diagram illustrating relevant waveforms of theDC-DC converter of FIG. 15A.

FIG. 15C is circuit diagram of one embodiment of the DC-DC convertershown in shown in FIG. 15A.

FIG. 15D is circuit diagram of one embodiment of the DC-DC convertershown in shown in FIG. 15A.

FIG. 16A is circuit diagram of a DC-DC converter.

FIG. 16B is a timing diagram illustrating relevant waveforms of theDC-DC converter of FIG. 16A.

FIG. 16C is circuit diagram of one embodiment of the DC-DC convertershown in shown in FIG. 16A.

FIG. 16D is circuit diagram of one embodiment of the DC-DC convertershown in shown in FIG. 16A.

FIG. 17A is circuit diagram of a DC-DC converter.

FIG. 17B is a timing diagram illustrating relevant waveforms of theDC-DC converter of FIG. 17A.

FIG. 17C is circuit diagram of one embodiment of the DC-DC convertershown in shown in FIG. 17A.

FIG. 18 is circuit diagram of a DC-DC converter.

FIG. 19 is a schematic diagram of an example brushless motor driver.

FIG. 20 is a schematic diagram of an example isolated DC-DC converter

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

There are several considerations that should be taken into account indesigning DC-DC converters. Noise generation is one consideration.Compactness of the DC-DC converter is another consideration. Powerconsumption may be the most important consideration.

FIG. 1 illustrates an example DC-DC converter 100 for converting a DCsource voltage Vin to a DC output voltage Vout for powering a subcircuit load like a CPU. The source voltage Vin may be provided directlyor indirectly by a battery such as a rechargeable lithium ion battery.DC-DC converter 100 can vary the magnitudes of output voltage Vout andcurrent Iout to accommodate changing requirements of the load.

High-side transistor 101 and low-side transistor 102 are each coupled toan output inductor Lout, which in turn is coupled to the CPU via anoutput node 104 and an output capacitor Cout as shown. A pulse-widthmodulation (PWM) driver circuit with dead time control 106 (hereinafterPWM circuit 106) generates complimentary high-side and low-side squarewaves (not shown) that drive gates g of transistors 101 and 102,respectively. The pulses of the high-side and low-side square wavesactivate transistor 101 and transistor 102. When active, high-sidetransistor 101 and low-side transistor 102 transmit current. PWM circuit106 may include a level shifter that adds a DC voltage component to thehigh-side square wave. The high-side square wave has a pulse width oft1, while the low-side square wave has a pulse width of t2, which can bedifferent from t1. Both square waves have a frequency f that PWM circuit106 can vary. PWM circuit 106 can also vary the duty cycles of the highand low-side square waves.

High-side transistor 101 transmits current I1 to output node 104 viainductor Lout with each pulse of the high-side square wave, whilelow-transistor 102 transmits current I2 from ground to output node 104via inductor Lout with each pulse of the low-side square wave. Since thehigh-side and low-side square waves are complimentary, which means theydo not overlap, only one of the transistors should be activated at anygiven time. Additionally, PWM circuit 106 introduces a dead time betweenpulses of the high-side and low-side square waves to prevent currentshoot through, a condition in which both high-side transistor 101 andlow-side transistor 102 are fully or partially active, thus creating aconductive path between Vin and ground through which current can “shootthrough.”

Noise Reduction

DC-DC converters should be quiet; they should not generate excessiveelectrical and electromagnetic noise that adversely affects neighboringsub circuits. DC-DC converter 100 should be compact in size and powerefficient, especially when used in portable electronic devices. DC-DCconverters consume power in several different ways. For example, powercan be consumed when current is conducted between the source s and draind of active transistor 101 or active transistor 102. The amount of thisconductive power loss depends on the magnitude of Rds(on), theresistance that exists between the drain d and source s in transistor101 or transistor 102 when activated. Another loss affecting power isattributable to the current that is needed to switch transistor 101 ortransistor 102 between its active and inactive states. The amount ofpower loss, which is referred to as switching loss, may depend on manyfactors including the frequency f of the square waves and the magnitudeof stray capacitances in transistors 101 or transistor 102.

Transistors 101 and 102 in FIG. 1 are shown as conventional or lateralmetal oxide semiconductor field effect transistors (MOSFETs). DC-DCconvertors often employ trench metal oxide field effect transistors(MOSFET) due to their compactness and low active drain-to-sourceresistance Rds(on). A lower Rds(on) can reduce the power consumed byDC-DC converters as will be more fully described below. The remainingdisclosures will be primarily described with reference to non-isolated,DC-DC converters employing one or more trench MOSFETs, it beingunderstood that the present invention should not be limited thereto.

FIG. 2 illustrates a cross-sectional view of an example trench MOSFET200 that could be employed in a DC-DC convertor like that shown inFIG. 1. Transistor 200 includes several layers within a substrate. Moreparticularly, transistor 200 includes a highly doped drain layer 202 ofa first conductivity type (e.g., N+), a drift layer 204 of the firstconductivity type (e.g., N or N−), a relatively thin base layer 206 ofsecond conductivity type (e.g., P), and a highly doped source layer 208of the first conductivity type (e.g., N+). A trench is formed in thesubstrate, which has opposing side walls 210 and a bottom 212. It isnoted the trench can be formed with a cylindrically shaped wall.

A gate insulating region 214 and an electrically conductive gate 216 isformed in the trench. Region 214 includes an insulating material such assilicon dioxide that can electrically insulate gate 216 from surroundingcomponents such as the base layer 206 and drift layer 204. Gate 216 maybe formed from a conductive material such as polysilicon. A source 220and a drain 222 may also be formed of a conductive material. Source 220is in ohmic contact with source layer 208 and base layer 206, whiledrain 222 is in ohmic contact with drain layer 202. Base layer 206 is inohmic contact with source layer 208 and drift layer 204, which in turnis in ohmic contact with drain layer 202. An insulating region 224isolates source 220 from gate 216. Insulating region 224, likeinsulating region 214, may be formed from an insulating material such assilicon dioxide.

In operation, MOSFET 200 can be activated when gate 216 reaches anappropriate voltage (i.e., a threshold voltage Vt). When activated, aconductive N-type inversion layer is formed in the P-type base layer206. The inversion layer electrically connects the N-type source anddrain regions 208 and 202, and allows for majority carrier conductiontherebetween. Because the gate 216 is separated from the base layer 206by an intervening insulating region 214, little if any gate current isrequired to maintain MOSFET 200 in the active or on state.

With continuing reference to FIG. 1, Cout and Lout can be reduced insize if transistor 101 and transistor 102 are switched at highfrequencies. While smaller Cout and Lout leads to more compact DC-DCconverters, the higher switching frequencies may lead to higherswitching losses in transistors 101 and 102. Additionally, higherswitching frequencies may lead to increased electrical orelectromagnetic noise that can adversely affect the CPU and/orneighboring electrical components including those that are directly orindirectly coupled to Vin. When used as a high-side transistor or alow-side transistor, trench MOSFET 200 may have a lower Rds(on), whichin turn may reduce its conduction losses. However, MOSFET 200 maycontain relatively higher stray capacitances, which may exacerbateswitching losses and noise generation.

FIG. 3 illustrates an alternative trench MOSFET 300 that could beemployed in DC-DC convertors like that shown in FIG. 1 or other devices.Trench MOSFET 300 is similar in many regards to other trench MOSFETSlike trench MOSFET 200 shown in FIG. 2. For example, trench MOSFET 300includes the same substrate layers 202-208 that are shown within FIG. 2.However, at least one significant difference exists between the trenchMOSFET 200 and trench MOSFET 300; trench MOSFET 300 includes anadditional trench element 302 that is positioned between gate 216 andtrench bottom 212. Trench element 302 is isolated from gate 216, drain222 and source 220. Trench element 302 can be formed of the sameconductive material (e.g., polysilicon) that is used to form gate 216.Insulating region 214 electrically isolates trench element 302 from gate216. In this configuration, trench element 302 shields gate 216 fromdrain 222.

With continuing reference to FIG. 3, the dimensions of MOSFET 300 may beselected to reduce resistive, capacitive effect and othercharacteristics, which may be important for reducing switching losses,conductive losses, etc. For example, the lateral thickness G of theinsulating region (i.e., the gate oxide) between gate 216 and base layer206 should be less than the lateral thickness H of the insulating regionbetween the wall of the drift region and the wall of trench element 302(i.e., the wall oxide of trench element 302), which may reduce Rds(on)and Coss (Coss=Cgd+Cds) of MOSFET 300. The length A from the channelbottom to the bottom of gate 216 should be less than B, the length fromthe bottom of source layer 208 to the bottom of the channel, which mayreduce Crss, the reverse transfer capacitance of MOSFET 300. A reductionof Coss and Crss may reduce switching losses. The width C of opposingsidewalls of the element trench may be less than the width D of opposingsidewalls of the gate trench, which may reduce Rds(on) and the currentdensity of in the drift region adjacent to element 302. The length E ofelement 302 should be equal to or longer than F, the length of gate 216,which may reduce noise generated by MOSFET 300 when subjected to aswitching waveform. The thickness I of the insulating region between thegate 216 and element 302 should be greater than G, the thickness of theinsulating region between gate 216 and base layer 206, which may reducethe gate charge Qg of MOSFET 300.

FIG. 4 illustrates a circuit diagram equivalent of MOSFET 300 shownwithin FIG. 3, including representations of stray capacitors therein.While traditional trench MOSFETS have three terminals (gate terminal g,source terminal s, and drain terminal d), trench MOSFET has a fourthterminal e for the trench element 302. Cgs represents the capacitancethat exists between the gate g and source s, Cds represents thecapacitance at the PN junction of the drain d and source s, and Cedrepresents the capacitance that exists between trench element e anddrain d. As noted above, the trench element e shields gate g from draind. A capacitance Cgd (not shown) exists between the gate g and drain d;however, this capacitance is very small as a result of the shieldingeffect of trench element e.

FIG. 5 illustrates a DC-DC converter 500 similar to DC-DC convertor 100in FIG. 1. DC-DC converter 500 includes a high-side transistor 501 and alow-side transistor 502, both of which take form in trench MOSFETs 200shown in FIG. 2. Additionally, DC-DC circuit 500 shows inductor LP1,which may take form in stray inductance in a printed circuit boardconnection between capacitor Cin and high-side transistor 501, andinductor LP2, which may take form in stray inductance in the printedcircuit board connection between low-side transistor 502 and ground.

PWM circuit 106 generates complimentary, high-side and low-side squarewaves (not shown) that drive the gates g of high-side and low-sidetransistors 501 and 502, respectively. Vgs, the voltage between the gateg and source s, of high-side and low-side transistors 501 and 502 riseand fall with the square waves. FIG. 6 illustrates several wave forms,including wave forms for Vgs at high-side and low-side transistors 501and 502. PWM circuit 106 introduces a “dead time” between pulses of thehigh-side and low-side square waves. This dead time minimizes currentshoot-through from Vin to ground, which can occur when both high-sidetransistor 501 and low-side transistor 502 are either fully or partiallyactivated. The dead time can be seen in the wave forms for Vgs in FIG.6. During the dead time output inductor Lout drives a forwarding currentIF through body diode of low-side transistor 502. The path taken by IFgoes through ground as shown in FIG. 5. FIG. 6 includes a waveform thatrepresents the forwarding current IF through the body diode of low-sidetransistor 502. As high-side Vgs transitions, a reverse recovery currentin is induced in the body diode of transistor 502. The path of reverserecovery current in goes through ground, LP1 and high-side transistor501 as shown in FIG. 5. In flows in the opposite direction of IF. Irrspikes (i.e., in is short in duration) with a peak value thatapproximates the magnitude of the drain current of the high-sidetransistor during the high-side pulse. Unfortunately, the spike createsa large ripple voltage with high frequency components on the Vin linevia LP1. The high frequency components may create electrical and/orelectromagnetic noise that can adversely affect nearby circuitsincluding those circuits that are connected to Vin either directly orindirectly.

FIG. 7 illustrates an alternative DC-DC convertor 700 that is similar toDC-DC convertor 500. DC-DC convertor 700 includes a high-side transistor701 and a low-side transistor 702, both of which take form in trenchMOSFETs like that shown in FIGS. 3 and 4. High-side transistor 701 andlow-side transistor 702 include trench elements e coupled together andto ground as shown.

With continuing reference to FIG. 7, FIG. 8 illustrates wave formsrelevant to operational aspects of the DC-DC converter 700. PWM circuit106 generates complimentary, high-side and low-side square waves (notshown), which in turn drive the gates g of high-side and low-sidetransistors 701 and 702, respectively. PWM circuit 106 introduces a deadtime between pulses of the high-side and high-side square waves toprevent current shoot through. Output inductor Lout drives forwardingcurrent IF, whose path is shown in FIG. 7, through body diode oftransistor 702 during the dead time. As high-side Vgs transitions, areverse recovery current in is induced in the body diode of transistor702 in the opposite direction of IF. Irr spikes as shown in FIG. 8.However, the reverse recovery current in does not flow through LP1.Rather, some or most of reverse recovery current in flows through Ced ofhigh-side transistor 701, which offers little resistance to ground athigh frequencies. As such, irr spike voltage disturbances on the Vinline are substantially reduced as shown in FIG. 8. This in turn willreduce adverse effects of electrical and/or electromagnetic noisegenerated by the DC-DC converter shown within FIG. 7.

FIG. 9 shows another DC-DC convertor 900 that is substantially similarto the DC-DC converter 700 shown in FIG. 7. DC-DC converter 900 includesa resistor Rx coupled between ground and the trench electrode oflow-side transistor 702. In one embodiment, Rx can be an externalcomponent, e.g., a component that is external to a substrate in which isformed high-side and low-side transistors 701 and 702, or an internalcomponent, e.g., a component that is internal to a substrate in which isformed high-side and low-side transistors 701 and 702 (i.e., ainterconnection resistance between trench element 302 of transistor 702and surface conductive layer). FIG. 9 also shows inductor LP3, which maytake form in stray inductance in the printed circuit board connectionbetween the source of hi-side transistor 701 and the switching node. Byconnecting the trench element of low-side transistor 702 to ground viaresistor Rx, a snubber circuit is formed. A snubber circuit is a deviceused to suppress voltage transients. During operation of the DC-DCconvertor 700 of FIG. 7, a spike voltage may occur at the switching nodewhen high-side transistor 701 activates due to the high rate of changeof current flowing through inductors LP1, LP2, and/or LP3. The spikevoltage at the switching node may produce adverse effects such asexceeding the breakdown voltage BVds of transistor 702. This voltagespike can be reduced by the snubber circuit that includes resistor Rxand the capacitance Ced that exists between the trench element and drainin low-side transistor 702.

FIG. 10 illustrates the wave form at the switching node for the DC-DCconverter 700 without the snubber circuit and the DC-DC converter 900with the snubber circuit. As can be seen, the snubber circuit acts toreduce the transient voltages at the switching node, which in turnreduces the chances that the breakdown voltage BVds is exceeded.

Power Loss Reduction

Voltage spikes and transients are some of the issues that should beconsidered when designing DC-DC converters. As noted above, power lossis another issue, especially in DC-DC converters employed in batterypowered devices. In general, a transistor consumes power when itconducts current between its source and drain, or when the transistorswitches between the active and inactive states. To illustrate, power islost by low-side transistor 702 of FIG. 7 when active and conductingcurrent. This conduction loss is dependent on Rds(on), the resistancebetween the source and drain. Power is also lost by low-side transistor702 when it switches between the active and inactive states in responseto changes in Vgs. This switching loss is dependent on at least threefactors: (1) tr, the time it takes Vgs to rise to the voltage of thepulse generated by PWM circuit 106 (see, e.g., FIG. 7); (2) tf, the timeit takes for Vgs to fall after the gate is driven to ground by PWMcircuit 106, and; (3) Coss=Cgd+Cds. It is noted that Cgd is small inlow-side transistor 702 as a result of shielding by the trench elemente. Because Cgd is small, Coss is approximately equal to Cds in low-sidetransistor 702. If Rds(on) can be lowered, conduction loss in low-sidetransistor 702 can be reduced accordingly. Further, if tr, tf and/orCoss can be lowered, switching loss in low-side transistor 702 can bereduced accordingly.

FIGS. 11, 12A, 13, and 14 illustrate circuits employing a trench MOSFET1100 like that shown within FIG. 4. For ease of illustration, the straycapacitances Cgs, Cds, and Ced are not shown in FIGS. 11, 12A, 13, and14. In FIG. 11, the gate g of trench MOSFET 1100 is set to 0 volts,which biases MOSFET 1100 in the inactive or off state. As Ves decreasesfrom a higher voltage to a lower voltage, Coss decreases. For example,when Ves decreases from +2 volts to −2 volts, Coss can decrease by 20%.This change in Coss may be attributable to a modulation of the depletionlayer thickness within the drift region of trench MOSFET 1100. It isalso noted that Cgd, although negligible, can decrease by 34% as Vesdecreases from +2 volts to −2 volts.

If the high-side and low-side transistors of a DC-DC converter areimplemented as trench MOSFETs like that shown in FIGS. 3 and 4 (e.g.,trench MOSFETs with trench elements 302), the shielding effect of thetrench elements may reduce switching losses when compared to theswitching losses of high-side and low-side transistors implemented astrench MOSFETs like that shown in FIG. 2. At the very least a decreasein Coss should result in a corresponding reduction in switching losses.

FIG. 12A illustrates MOSFET 1100 with its gate g tied to a source thatgenerates a square wave like the low-side square wave generated by PWMcircuit 106 in FIGS. 7 and 9. The square wave varies between 0 volts and5 volts. Additionally, the drain of MOSFET 1100 is coupled to a load Lin series with a voltage source as shown. In this configuration, trenchMOSFET 1100 switches between the active and inactive states as thesquare wave changes between 0 and 5 volts. Vgs doesn't instantly changewith the transitions of the square wave. To illustrate, FIG. 12B showsseveral wave forms: Vgs with Ves set to 2 volts; Vgs with Ves set to 0volts, and; Vgs with Ves to −2 volts. Tf and tr, the fall and risetimes, respectively of Vgs, decreases when Ves is set to a lowervoltage. FIG. 12B shows tf and tr for Vgs with Ves set to 0 volts. Adecrease in tr and tf should result in corresponding reductions inswitching losses.

FIG. 13 illustrates trench MOSFET 1100 with Vgs set to 5 volts, whichbiases MOSFET 1100 to the active or on state. As Ves increases from alower voltage to a higher voltage, Rds(on) decreases. For example, asVes increases from −2 volts to +2 volts, Rds(on) may decrease by 6%. Thedecrease in Rds(on) may be attributable to an accumulation of electroncharge in the drift zone, which in turn may reduce drift zoneresistance.

FIG. 14 illustrates the trench MOSFET 1100 shown in FIG. 13 with atrench element control circuit coupled to trench element e. Trenchelement control circuit can dynamically control the voltage applied totrench element e in order to reduce power losses in trench MOSFET 1100.In one embodiment, trench element control circuit applies one of aplurality of voltages to trench element e depending on the state oftrench MOSFET 1100. For example, trench element control circuit mayapply a first voltage to trench element e while trench MOSFET 1100 isswitching between the active and inactive states in order to reduceswitching losses, and trench element control circuit may apply a second,higher voltage to trench element e when trench MOSFET 1100 is active inorder to reduce conduction losses.

In one embodiment, trench element control circuit can apply one of aplurality of voltages to trench element e based on the magnitude of Vgs.For example, trench element control circuit may apply a first voltage(e.g., −4.3 volts) to trench element e when Vgs is less than a thresholdvoltage Vc (e.g., 4 volts), and trench element control circuit may applya second voltage (e.g., 5 volts) to trench element e when Vgs is greaterthan the threshold voltage Vc. The threshold voltage Vc can bepreselected based on the voltage of the pulse generated by the squarewave generator. In one embodiment, the threshold voltage may be set to75-95% of the pulse voltage magnitude, it being understood the presentinvention should not be limited thereto.

FIGS. 15A, 16A, and 17A illustrate three different embodiments of aDC-DC converter employing at least one trench MOSFET like that shown inFIGS. 3 and 4, and at least one trench element control circuit like thatshown within FIG. 14. For ease of illustration, the stray capacitancesCgs, Cds, and Ced are not shown in FIGS. 15A, 16A, and 17A.

In FIG. 15A, the high-side transistor 1501 is generically illustratedand may take form in a lateral MOSFET like that shown in FIG. 1, atrench MOSFET like that shown in FIG. 2, or a trench MOSFET like thatshown within FIGS. 3 and 4. Low-side transistor 1502 takes form in atrench MOSFET like that shown in FIGS. 3 and 4. The trench elementcontrol circuit controls Ves based on Vgs. The trench element controlcircuit applies a first voltage Vl1 (e.g., −4.3 volts) to element e whenVgs is less than a threshold voltage Vlt (e.g., 4 volts), and trenchelement control circuit applies a second, higher voltage Vl2 (e.g., 5volts) to element e when Vgs exceeds the threshold voltage Vlt. FIG. 15Bshow three waveforms; one for the low-side square wave generated by PWMcircuit 106, a waveform for Vgs, and a waveform for Ves. PWM circuit 106drives gate g of the low-side transistor 1502 via the low-side squarewave.

The trench element control circuit monitors Vgs as it follows thelow-side square wave. The trench element control circuit applies firstvoltage Vl1 to element e soon after Vgs begins its transition from 5volts to zero volts, and the trench element control circuit applies thesecond, larger voltage Vl2 to the element e just before Vgs rises to 5volts. One of ordinary skill understands that when Vgs is greater thanthe threshold voltage Vlt (e.g., 4 volts) while Ves is set to the secondvoltage Vl2, low-side transistor 1502 is active and loses less powerthrough conduction when compared to the conduction loss when low-sidetransistor 1502 is active while Ves is set to the first, lower voltageVl1. Further, when Vgs is less than the threshold voltage Vlt while Vesis set to the first voltage Vl1, low-side transistor 1502 is inactive orin transition between the active and inactive states. While intransition between the active and inactive states, low-side transistor1502 should lose less power through switching when compared to theswitching loss experienced while Ves is set to the second voltage Vl2.

In FIG. 16A, high-side transistor 1601 takes form in a trench MOSFETlike that shown within FIGS. 3 and 4. The trench element control circuitof FIG. 16A is similar to the trench element control circuit of FIG.15B. The low-side transistor 1602 in FIG. 16A is generically shown andmay take form in a lateral MOSFET like that shown within FIG. 1, atrench MOSFET like that shown in FIG. 2, or the trench MOSFET like thatshown in FIGS. 3 and 4. In this embodiment, the trench element controlcircuit controls the voltage applied to element e of the high-sidetransistor 1601 based on the voltage at the gate g. For example, thetrench element control circuit applies a first voltage Vh1 to element ewhen the gate voltage is less than a threshold voltage Vht, and trenchelement control circuit applies a second, higher voltage Vh2 to elemente when the gate voltage is greater than the threshold voltage Vth. FIG.16B shows separate waveforms for high-side square wave generated by PWMcircuit 106, Vgs, and Ves. PWM circuit 106 drives gate g of thehigh-side transistor 1601 via the high-side square wave.

The trench element control circuit monitors the voltage at the gate g asit changes with the change in the high-side square wave. The trenchelement control circuit applies the first voltage Vh1 soon after thegate voltage begins its transition from the high voltage to the lowvoltage, and the trench element control circuit applies the second,larger voltage Vh2 just before gate voltage rises to the high voltage.When the gate voltage is greater than the threshold voltage Vht whileelement e is set to the second voltage Vh2, high-side transistor 1601should be active and should lose less power through conduction whencompared to the conduction loss when high-side transistor 1601 is activewhile the voltage at element e is set to the first voltage Vh1. When thevoltage at the gate is less than the threshold voltage Vht while thevoltage at element e is set to the first voltage Vh1, high-sidetransistor 1601 should be inactive or in transition between the activeand inactive states. While in transition between the active and inactivestates, high-side transistor 1601 should lose less power throughswitching when compared to the switching loss while element e is set tothe second voltage Vh2.

The DC-DC converter 1700 shown within FIG. 17A includes high-side andlow-side transistors 1701 and 1702 that take form in trench MOSFETs likethat shown within FIGS. 3 and 4. DC-DC converter 1700 also includeshigh-side and low-side trench element control circuits that are similarto the trench element control circuits in FIGS. 16B and 15B,respectively.

Trench element control circuit 1704 applies a first voltage Vh1 toelement e when the voltage at gate g is less than threshold voltage Vht,and trench element control circuit 1704 applies a second, higher voltageVh2 to element e when the gate voltage is greater than threshold voltageVht. FIG. 17B shows waveforms for the square waves generated by PWMcircuit 106, Vgs, and Ves for low-side and high-side transistors. PWMcircuit 106 drives gate g of the high-side transistor 1701 via thehigh-side square wave.

Trench element control circuit 1704 monitors the voltage at gate g as itfollows the high-side square wave. Trench element control circuit 1704applies the first voltage Vh1 to element e soon after the voltage atgate g begins its transition from the high voltage to the low voltage,and trench element control circuit 1704 applies the second, largervoltage Vh2 to element e just before the gate voltage rises to the highvoltage.

Low-side trench element control circuit 1706 applies a first voltage Vl1(e.g., −4.3 volts) to element e when Vgs of low-side transistor 1702 isless than a threshold voltage Vlt (e.g., 4 volts), and trench elementcontrol circuit 1706 applies a second, higher voltage Vl2 (e.g., 5volts) to element e when Vgs exceeds the threshold voltage Vlt. Withcontinuing reference to FIG. 17B, trench element control circuit 1706controls the voltage at trench element e based on Vgs. PWM circuit 106drives gate g of the low-side transistor 1702 via the low-side squarewave. Trench element control circuit 1706 monitors Vgs as it follows thelow-side square wave.

Trench element control circuit 1706 applies the first voltage v11 toelement e soon after Vgs begins its transition from 5 volts to zerovolts, and trench element control circuit 1706 applies the second,larger voltage Vl2 to element e just before Vgs rises to 5 volts. Thefirst voltage Vh1 applied by trench element control circuit 1704 may begreater than the first voltage Vl1 applied by trench element controlcircuit 1706. Likewise, the second voltage Vh2 applied by trench elementcontrol circuit 1704 may be greater than the second voltage Vl2 appliedby trench element control circuit 1706.

FIGS. 15C and 15D illustrate alternative embodiments of the DC-DCconverter shown within FIG. 15A. In FIG. 15C, high-side transistor 1501takes form in a trench MOSFET like that shown within FIG. 2. The trenchelement control circuit includes a comparator 1510 that compareslow-side Vgs with Vlt=4 volts, which is provided by a constant source.When low-side Vgs rises above 4 volts, comparator 1510 causes switch1512 to couple trench element e to a source that provides Vl2=5.0 volts.In other words, as low-side transistor 1502 activates, the trenchelement control circuit couples element e to 5 volts, which in turnreduces the conduction loss of low-side transistor 1502 while it isactive. However, as low-side Vgs begins to fall below Vlt=4 volts,comparator 1510 causes switch 1512 to couple element e to a source thatprovides Vl1=−4.3 volts. In other words, as the low-side transistor 1502begins to transition between the active and inactive states, trenchelement e is coupled to −4.3 volts, which reduces switching loss withinlow-side transistor 1502.

DC-DC converter 1500 shown within FIG. 15D includes the same trenchelement control circuit and low-side transistor found within FIG. 15C.However, the high-side transistor 1501 takes form in a trench MOSFETlike that shown within FIGS. 3 and 4. In this configuration, trenchelement e of the high-side transistor 1501 is coupled to ground. In thisconfiguration, any voltage spikes caused by in (described above withreference to FIG. 8) should be reduced or eliminated.

FIGS. 16C and 16D illustrate alternative embodiments of the DC-DCconverter 1600 shown within FIG. 16A. In FIG. 16C, low-side transistor1602 takes form in a trench MOSFET like that shown within FIG. 2. Thetrench element control circuit includes a comparator 1610 that comparesthe voltage at gate g with Vht provided by a constant source. Whenhigh-side Vgs rises above Vht, comparator 1610 causes switch 1612 tocouple trench element e to a source that provides Vh2. In other words,as high-side transistor 1602 activates, the trench element controlcircuit couples element e to Vh2 volts, which in turn should reduce theconduction loss of high-side transistor 1602 while it is active.However, as the voltage at gate g begins to fall below Vht, comparator1610 causes switch 1612 to couple element e to a source that providesVh1. In other words, as the high-side transistor 1602 begins totransition between the active and inactive states, trench element e iscoupled to Vh1, which should reduce switching loss within high-sidetransistor 1602.

DC-DC converter 1600 shown within FIG. 16D includes the same trenchelement control circuit and high-side transistor found within FIG. 16C.However, the low-side transistor 1602 takes form in a trench MOSFET likethat shown within FIGS. 3 and 4. In this configuration, a snubberresistor Rx is coupled between trench element e and ground. In thisconfiguration, any voltage spikes at the switching node should bereduced as described above with reference to FIG. 10.

FIG. 17C illustrates an embodiment of the DC-DC converter 1700 shownwithin FIG. 17A. The trench element control circuit 1704 includes acomparator 1710 that compares the voltage at gate g with Vht provided bya constant source. When the voltage at the gate g rises above Vht,comparator 1710 causes switch 1712 to couple trench element e to asource that provides Vh2. In other words, as high-side transistor 1702activates, the trench element control circuit 1704 couples element e toVh2, which in turn reduces the conduction losses of high-side transistor1702 while it is active. However, as the voltage at gate g begins tofall below Vht, comparator 1710 causes switch 1712 to couple element eto a source that provides Vh1. In other words, as the high-sidetransistor 1702 begins to transition between the active and inactivestates, trench element e is coupled to Vh1, which reduces switchinglosses within high-side transistor 1702.

The low-side trench element control 1706 circuit includes a comparator1714 that compares low-side Vgs with Vlt=4 volts provided by a constantsource. When low-side Vgs rises above 4 volts, comparator 1714 causesswitch 1714 to couple trench element e to a source that provides Vl2=5.0volts. In other words, as low-side transistor 1702 activates, the trenchelement control circuit 1706 couples element e to 5 volts, which in turnreduces conduction losses of low-side transistor 1702 while it isactive. However, as low-side Vgs begins to fall below 4 volts,comparator 1714 causes switch 1716 to couple element e to a source thatprovides Vl1=−4.3 volts. In other words, as the low-side transistor 1702begins to transition between the active and inactive states, trenchelement e is coupled to −4.3 volts, which reduces switching losseswithin low-side transistor 1702.

FIG. 18 illustrates relevant components of one embodiment of the DC-DCconverter shown within FIG. 15A. Many of the components in FIG. 15A arecomponents that are not shown in the prior Figures, but are contained inthe PWM circuit 106. More particularly, FIG. 18 shows a level shifterand dead time control circuit 1802, a dead time control circuit 1804,and inverter 1806 of the PWM circuit 106. Additionally, a square wavegenerator 1808 of PWM circuit 106 is also shown. Square wave generator1808 generates a square wave that is provided to level shifter and deadtime control circuit 1802 and the series combination of inverter 1806and dead time control circuit 1804, which in turn generate thecomplementary high-side and low-side square waves, respectively,described above.

FIG. 18 additionally shows a source 1810 that generates first voltageVl1. As shown, source 1810 includes diode 1812 coupled between diode1814 and capacitor 1816. Additionally, a capacitor 1818 is coupledbetween diode 1814 and inverter 1820 as shown. An input to inverter 1820is coupled to the square wave output provided by generator 1808. Source1810 generates a constant first voltage Vl11=−4.3 volts for the trenchelectrode control circuit. It is noted the trench electrode controlcircuits shown within FIGS. 16A and 16B may take form in the trenchelement control circuit shown within FIG. 18.

Additional Devices Using the New Trench MOSFET

The foregoing describes use of a trench MOSFET of FIGS. 3 and 4 inexample non-isolated, DC-DC converters. The trench MOSFET can beemployed in other devices. FIG. 19 is a schematic diagram of an examplebrushless motor driver 1900 employing trench MOSFETS like that shown inFIGS. 3 and 4. Driver 1900 includes high-side transistors 1901-1903coupled to respective low-side transistors 1904-1906, each of which maytake form in the trench MOSFET shown within FIGS. 3 and 4. Trenchelement control circuits (TECC) 1911-1913 are coupled to and controlhigh-side transistors 1901-1903, while trench electrode control circuits1914-1916 are coupled to and control low-side transistors 1904-1906,respectively. The trench electrode control circuits 1911-1916 may takeform in the trench electrode control circuits shown in FIG. 14 or 17C.These trench electrode control circuits operate to reduce the conductionand switching losses transistors 1901-1906.

High side driver circuit 1908 generates square waves that driverespective gates of high-side transistors 1901-1903. Likewise, low-sidedriver circuit 1910 generates square waves that drive respective gateslow-side transistors 1904-1906. The duty cycles of the square wavesgenerated by the high-side driver 1908 and low-side driver 1910 may becontrolled based upon inputs thereto. Like the trench element controlcircuits shown within FIG. 14 or 17C, each of the trench element controlcircuits 1911-1916 controls the voltage applied to the trench elements eof transistors 1901-1906, respectively, based on the voltage at thegates g of transistors 1901-1906. For example, trench element controlcircuits 1911-1916 may apply a first voltage or a second voltage totrench elements e of transistors 1901-1906, respectively. The firstvoltage applied to elements e of transistors 1901-1903 may differ fromthe first voltage applied to elements e of transistors 1904-1906, andthe second voltage applied to elements e of transistors 1901-1903 maydiffer from the second voltage applied to elements e of transistors1904-1906. The first voltage is greater than the second voltage. Thevoltage applied to element e depends on the voltage at gate g. Forexample, when the voltage at gate g of transistors 1901-1906,respectively, is less than the threshold voltage Vt of transistors1901-1906, trench element control circuits 1911-1916 may apply thesecond voltage to trench elements e of transistors 1901-1906,respectively. And when the voltage at gate g transistors 1901-1906,respectively, is greater than the threshold voltage Vt of transistors1901-1906, trench element control circuits 1911-1916 may apply the firstvoltage to trench elements e of transistors 1901-1906, respectively. Inthis fashion, trench element control circuits 1911-1916 can reduce theconduction and switching losses of transistors 1901-1906, respectively.Ultimately, transistors 1901-1906 coils 1921-1923 via nodes U, V, and W.Coils 1921-1923 generate a 3-phase rotating magnetic field that moves amotor rotor (not shown).

FIG. 20 is a schematic diagram of an example isolated DC-DC converteremploying trench MOSFETS like that shown within FIGS. 3 and 4. UnlikeDC-DC converters described above, the example isolated DC-DC converter2000 shown within FIG. 20 separates the input voltage Vin from theoutput voltage Vout using a transformer T. Converter 200 has full bridgedriver in a primary driving system and synchronous switching for asecondary system.

Convertor 2000 includes high-side transistors 2001 and 2002 coupled torespective low-side transistors 2004 and 2005, each of which may takeform in the trench MOSFET shown within FIGS. 3 and 4. Trench elementcontrol circuits (TECC) 2011-2012 are coupled to and control high-sidetransistors 2001 and 2002, while trench electrode control circuits2014-2015 are coupled to and control low-side transistors 2004 and 2005,respectively. Synchronous switching transistors 2003 and 2006 take formin the trench MOSFETs shown within FIGS. 3 and 4. TECC 2013 and 2016 arecoupled to and control synchronous switching. The trench electrodecontrol circuits 2011-2016 may take form in the trench electrode controlcircuits shown in FIG. 14 or 17C. These trench electrode controlcircuits operate to reduce the conduction and switching losses intransistors 2001-2006.

High side driver circuit 2008 generates square waves that driverespective gates of high-side transistors 2001 and 2002. Likewise,low-side driver circuit 2010 generates square waves that driverespective gates low-side transistors 2004 and 2005. The duty cycles ofthese square waves generated by the high-side driver 2008 and low-sidedriver 2010 may be controlled based upon inputs provided by primarysquare wave control circuit 2020. A secondary square wave controlcircuit generates square waves that drive respective gates oftransistors 2003 and 2006.

Like the trench element control circuits shown within FIG. 14 or 17C,each of the trench element control circuits 2011-2016 controls thevoltage applied to the trench elements e of transistors 2001-2006,respectively, based on the voltage at the gates g of transistors2001-2006. For example, trench element control circuits 2011-2016 mayapply a first voltage or a second voltage to trench elements e oftransistors 2001-2006, respectively. The first voltage is greater thanthe second voltage. The voltage applied to element e depends on thevoltage at gate g. For example, when the voltage at gate g oftransistors 2001-2006, respectively, is less than the threshold voltageVt of transistors 2001-2006, trench element control circuits 2011-2016may apply the second voltage to trench elements e of transistors2001-2006, respectively. And when the voltage at gate g transistors2001-2006, respectively, is greater than the threshold voltage Vt oftransistors 2001-2006, trench element control circuits 2011-2016 mayapply the first voltage to trench elements e of transistors 2001-2006,respectively. In this fashion, trench element control circuits 2011-2016can reduce the conduction and switching losses of transistors 2001-2006,respectively.

Although the present invention has been described in connection withseveral embodiments, the invention is not intended to be limited to thespecific forms set forth herein. On the contrary, it is intended tocover such alternatives, modifications, and equivalents as can bereasonably included within the scope of the invention as defined by theappended claims.

What is claimed is:
 1. An apparatus comprising: a first transistorcomprising: a first trench formed in a first semiconductor substrate; afirst source; a first drain; a first gate; a first conductive element inthe first trench; wherein the first conductive element extends betweenthe first gate and a bottom of the first trench; wherein the firstconductive element is isolated from the first source and the first gateby an insulating material; a circuit for generating a first pulse widthmodulated (PWM) signal at a first output, wherein the first output ofthe circuit is electrically coupled to the first gate.
 2. The apparatusof claim 1 further comprising: a second transistor comprising: a secondtrench formed in a second semiconductor substrate; a second source; asecond drain; a second gate; a second conductive element in the secondtrench; wherein the second conductive element extends between the secondgate and a bottom of the second trench; wherein the second conductiveelement is isolated from the second source and the second gate by aninsulating material; wherein the circuit is configured to generate asecond PWM signal at a second output, wherein the second output of thecircuit is electrically coupled to the second gate.
 3. The apparatus ofclaim 2 further comprising: an inductor comprising first and secondterminals; wherein the first source and the second drain areelectrically coupled together and to the first terminal of the inductor,and; wherein the first and second conductive elements are electricallycoupled together.
 4. The apparatus of claim 3 further comprising: aresistor comprising first and second terminals; wherein the first andsecond conductive elements are electrically coupled to the first andsecond terminals of the resistor, respectively.
 5. The apparatus ofclaim 1 wherein the first semiconductor substrate comprises a firstdrain layer, a first drift layer, a first base layer, and a first sourcelayer, wherein the first drift and base layers extend between the firstdrain and source layers, wherein the first base layer extends betweenthe first source drift layers, wherein the first source and base layersare in ohmic contact with the first source, and wherein the first drainlayer is in ohmic contact with the first drain, and wherein the firstconductive element extends between the first gate and the first drainlayer.
 6. The apparatus of claim 5 wherein the first trench extends intothe first source, base, and drift layers, but not into the first drainlayer, and wherein the first gate is adjacent the first base layer andis configured to influence the conductance thereof.
 7. The apparatus ofclaim 1 wherein a lateral thickness of insulating material between thefirst gate and the trench is less than a lateral thickness of insulatingmaterial between the first conductive element and the trench.
 8. Theapparatus of claim 7 wherein a distance between opposing sidewalls ofthe trench adjacent the first gate is greater than a distance betweenopposing sidewalls of the trench adjacent the first conductive element.9. The apparatus of claim 7 wherein a cross sectional width of the firstgate is greater than a cross sectional width of the first conductiveelement.
 10. The apparatus of claim 1 wherein a length of the first gateis equal to or greater than a length of the first conductive element.11. The apparatus of claim 1 wherein a width of insulating materialbetween the first gate and the first conductive element is greater thana lateral thickness of insulating material between the first gate andthe trench.
 12. The apparatus of claim 1 wherein the first transistorcomprises a base layer extending between a source layer and a driftlayer, wherein the trench extends through each of the source, base, anddrift layers, wherein a length of the base layer that extends betweenthe source layer and the drift layer is greater than an extension of thefirst gate past a boundary between the base layer and the drift layer.13. The apparatus claim 1 further comprising a first circuit, whereinthe first circuit is configured to couple the first conductive elementto a first voltage while the first transistor is in an active state. 14.The apparatus of claim 13 wherein the first circuit is configured tocouple the first conductive element to a second voltage while the firsttransistor is in an inactive state, and wherein the first voltage isdifferent than the second voltage.
 15. The apparatus of claim 12 whereinthe first circuit is configured to decouple the first conductive elementfrom the first voltage as the first transistor transitions from theactive state to the inactive state, and wherein the first circuit isconfigured to couple the first conductive element to the second voltageas the first transistor transitions from the active state to theinactive state.
 16. The apparatus of claim 14 wherein the first circuitis configured to decouple the first conductive element from the secondvoltage as the first transistor transitions from the inactive state tothe active state, and wherein the first circuit is configured to couplethe first conductive element to the first voltage as the firsttransistor transitions from the inactive state to the active state. 17.The apparatus of claim 2 wherein the first and second PWM signals arecomplementary and non-overlapping.
 18. The apparatus of claim 1 furthercomprising: a second transistor comprising: a second trench formed in asecond semiconductor substrate; a second source; a second drain; asecond gate; a second conductive element in the second trench; whereinthe second conductive element extends between the second gate and abottom of the second trench; wherein the second conductive element isisolated from the second source and the second gate by an insulatingmaterial; wherein the first and second drains are electrically coupledto each other; wherein the circuit is configured to generate a secondPWM signal at a second output, wherein the second output of the circuitis electrically coupled to the second gate.
 19. The apparatus of claim18 further comprising an inductor comprising first and second terminals;wherein the first and second sources are electrically coupled to thefirst and second terminals, respectively, of the inductor.
 20. Theapparatus of claim 19 further comprising: first and second inductors;wherein the first and second sources are electrically coupled to thefirst and second inductors, respectively.